This invention relates to digital scan converters of the type that convert analog signal information that is referenced to a polar coordinate system into signals for activating a display device that operates in a Cartesian coordinate format. More specifically, this invention relates to improvements in the type of digital scan converters which effect polar-to-Cartesian coordinate conversion by a signal processing technique wherein: (a) a first portion of the conversion process transforms analog image information that is a function of polar coordinate vectors (r and .theta.) into a set of stored digital signals that are functions of one polar coordinate vector and one Cartesian coordinate vector (e.g., each stored signal is a function of .theta. and y); and (b) the final portion of the conversion process is effected as the stored digital signals are read from the system memory and converted to a signal that is compatible with a conventional raster scan television system or another type of Cartesian format display device.
As is known in the art, various surveillance apparatus such as slow scan radar, ultrasonic imaging and sonar systems scan a sectorial region of an object or the surrounding environment by emitting a series of energy pulses which propagate outwardly along angularly spaced apart, radially directed paths and by detecting return or echo signals that occur when the emitted pulses are scattered by reflective structure that lies within the propagation paths. Since the time that elapses between the emission of a particular energy pulse and the associated return signal (or any portion thereof) is related to the distance between the structure causing the signal reflection and the source of the energy, a return signal can be processed to provide a two dimensional, visual representation of the scanned region. In this respect and regardless of the transmission techniques employed, the operation of such surveillance systems is equivalent to the rotation of the single-element transducer through a total scanning angle by repeatedly moving the transducer through small, incremental, angular steps while emitting a pulse of energy and receiving the associated return signal at each angular position of the transducer. Thus, the signals provided by sector scanning surveillance systems are inherently based on a polar coordinate system wherein the emitted energy can be mathematically modeled as orginating at a point source that is located at the origin of the polar coordinate system; the value of the angular or azimuthal coordinte, .theta., expresses the direction of an emitted energy pulse and the associated return signal; and the value of the radial coordinate, r, expresses the radial distance between the transducer and the structure causing the reflective energy scattering.
Although the image data of sector scanning surveillance systems is gathered or collected in polar format, the operation of most modern display devices is based on a Cartesian coordinate system in which the image being displayed is generated by rapidly and successively producing small incremental regions of illumination on the face of a cathode ray tube or a mosaic of electroilluminescent elements while modulating the intensity of the illumination in accordance with the amplitude vs. time characteristics of the surveillance system return signals. In this respect, in order to provide high image resolution, the incremental elements of such display devices must be relatively small and must be successively energized at a relatively high rate in order to update or refresh the visual display at a rate which provides "flicker free" viewing. The rate at which return signals are provided by a surveillance system may, of course, differ substantially from the rates at which flicker free visual displays can be produced and, in many situations, the display rate may be fixed by convention or by design constraints. For example, in accordance with the convention utilized in the United States and various other countries, the electron beam of a television display is swept horizontally across the face of the television screen at a rate of 15,750 traces per second while being swept from the top to the bottom of the screen at a 60 Hz rate. Therefore, each image frame is comprised of 525 horizontal traces with the electron bean travelling across the screen in approximately 55 microseconds and retracing to the original vertical border of the screen in approximately 8 microseconds. With reference to vertical displacement, the horizontal traces are interlaced with one another by effecting one half of the horizontal traces during one period of the 60 Hz vertical scanning rate and effecting the remaining sweeps in spatial alternation with the previously generated horizontal sweeps during the next period of the 60 Hz vertical scanning rate. Thus, the image being displayed by this type of television system is completely updated or refreshed at a rate of approximately 30 Hz.
Various digital scan converters have been proposed for accepting data at the rate at which it is made available by a surveillance system or other source that, in effect, operates with reference to a polar coordinate system and providing the data at a different rate while simultaneously facilitating display with a device that operates in a Cartesian coordinate system. Basically, such prior art digital scan converters sample each surveillance system reflection signal at a predetermined rate to provide a set of digitally encoded signals (digital words) that represent the amount of reflection occurring from spaced apart locations along the radially extending scanning path that was traversed by both the transmitted energy pulse and the associated return signal. Since, as was previously noted, sector scanning surveillance systems sequentially emit pulses along a plurality of angularly oriented scanning paths in order to provide a two-dimensional surveillance region of sectorial geometry, one complete scanning sequence produces a plurality of sets of digital words wherein each set of digital words represents reflections occurring along a particular scanning path and successive sets of digital words occur at the pulse repetition rate of the surveillance system. The sets of digital words are then stored in a memory device such as a random access memory (RAM) in a format which identifies each digital word with a corresponding spatial position within the scanned region. The stored digital words are then read from the system memory device at a rate compatible with the display apparatus of interest and are typically converted to a television-compatible analog signal or a signal suitable for use with other Cartesian coordinate display apparatus.
In the type of prior art scan converter that is most relevant to this invention, each reflection signal is digitized (sampled) at a rate proportional to the cosine of the azimuthal angle that defines the associated scanning path relative to a .theta.=0 axis that bisects the sectorial region imaged by the surveillance system. This causes the successive digital words produced for each scanning path to represent reflection occurring from small spatial regions that are formed or defined by the intersection of the current scanning path and a set of parallel, spaced apart traces that are perpendicular to the .theta.=0 axis. Thus, if the .theta.=0 axis of the polar system exhibits a constant x coordinate in Cartesian space, it can be recognized that digitizing the successive surveillance system reflection signals in this manner provides a collection of digital signals wherein each signal represents reflection occurring at a location most easily defined in terms of the .theta. coordinate value of the original polar format and the y coordinate value of the desired Cartesian format.
The prior art includes various proposals for completing the polar-to-Cartesian coordinate conversion in scan converters of the above-described type, i.e., for converting the signal stored in memory on the basis of a .theta. and y coordinate to a signal associated with an x and y location. For example, U.S. Pat. No. 4,002,827, issued to Nevin, utilizes a random access memory (RAM) that can be considered to be a rectangular array that bears a one-to-one correspondence with rectangular or square spatial elements formed on the face of a display device such as a cathode-ray tube. As each digitized sample of a particular return signal is being derived in the above-indicated manner, the x coordinate associated therewith is calculated by determining the radial coordinate of the digital sample and multiplying that value by the sine of the azimuthal angle that defines the current scanning path. This signal is then utilized to determine the x address (column address) of the RAM storage location which receives the digital image information and the successive digital words associated with each return signal are supplied to RAM storage locations of successively increasing y addresses (row addresses). Since each row of the rectangular memory array can be associated with one complete horizontal line of the Cartesian display format, an analog signal compatible with conventional black and white television systems can be formed in the type of system proposed by the Nevin patent by successively reading the stored data on a row-by-row basis with each row of data being clocked to a digital-to-analog converter (DAC) at a constant rate that is dictated by the horizontal sweep rate of the television system.
One of the primary disadvantages and drawbacks of a system such as the type disclosed in the Nevin patent is that an extremely large memory is required in order to represent each small incremental region of the face of a television set or other Cartesian format display device. For example, in one embodiment of the Nevin scan converter for displaying a sectorial surveillance region on a television screen, the display region of the television screen is considered to comprise 512 horizontal traces consisting of 512 spaced apart "dots" so that the display is, in effect, an array containing 262,144 incremental regions. Thus, if the sampling technique utilized to digitize the surveillance system reflection signals results in four-bit digital words, a RAM having a 1-megabit storage capability is required even though relatively few of the storage locations will actually hold signal information. In particular, in displaying a sectorial surveillance region on the substantially rectangular face of a television screen, a major portion of the screen area will be outside of the display sector and hence the RAM storage locations corresponding to this region of the television screen are not utilized to store image information but are necessary so that the system DAC will supply an analog signal having the proper amplitude versus time characteristics when the stored data is read from memory at a constant clock rate.
Other prior art polar format to Cartesian format scan converters of the type being discussed overcome the requirement for an extremely large memory size that is encountered in the system proposed by Nevin by controlling the manner in which data is read from the RAM, both as to the time intervals in which data is read and as to the rate at which data is read. More specifically, U.S. Pat. No. 4,214,269 issued to Parker et al., and assigned to the assignee of this invention, and U.S. Pat. No. 4,245,250, issued to Tiemann, each discloses digital scan converter arrangements wherein the image memory is equivalent to a rectangular array of storage locations with the number of columns equal to the number of surveillance system scanning paths and the number of rows equal to the number of horizontal television traces that are to include image information (i.e., the number of television lines required to establish the vertical dimension of the display sector). To properly display the stored information, each row of stored image information is loaded into a buffer memory and sequentially clocked therefrom under the control of a conventional television horizontal sync system so that data is provided to the system DAC at a rate that varies inversely with the y coordinate that is associated with that particular row of data (i.e., data is clocked to the DAC at a rate that is inversely proportional to the width of the sectorial surveillance pattern at that particular position within the surveillance pattern so that image information associated with display traces near the apex of the display sector are read at a faster rate than image information associated with subsequent display traces. In addition, the systems disclosed in the Parker et al. and Tiemann patents include means for controlling the time at which data is read from the buffer memory so that the portions of the analog signal provided by the DAC which represent image information begin at the boundary edge of the sectorial display pattern. This requires that the first clock pulse that controls the transfer of data from buffer storage to and through the system DAC must be delayed relative to the television system horizontal sync signal by an amount of time that is a function of the y coordinate of the image being fromed and displayed.
Although the arrangements disclosed by the Parker et al. and the Tiemann patents greatly reduce the system storage requirements in that the RAM for storing the image information need only exhibit n columns of storage locations and m rows of storage locations, where the surveillance system employs n scanning paths and m rows (e.g., TV lines) contain image, other disadvantages and drawbacks are encountered. For example, in order to provide image information that is associated with locations that are equally spaced apart from one another relative to the x coordinate of the Cartesian display format, the system proposed in the Tiemann reference is limited to the use of scanning paths that are oriented so that the difference between the tangents of the azimuthal angles that define adjacent scanning paths remains constant throughout the entire surveillance region.
The system disclosed in the Parker et al. patent is not subject to the limitation associated with the system proposed by Tiemann since it includes an arrangement for modulating the pulse repetition rate of the signal which clocks data from the output buffer memory into the DAC at a rate which is a function of the column location of the data entry being read. In particular, in the system of the Parker et al. patent, the rate at which data is read from the output buffer memory not only varies on a row-by-row basis relative to the storage locations of the RAM but also is varied while reading a particular row of information so that the rate increases as successive column entries are read until the data entry being read corresponds to the .theta.=0 axis of the surveillance system and then decreases in a like manner until the last data entry is read.
Although the arrangement disclosed in the Parker et al. patent thus overcomes numerous prior art problems, certain areas remain open for improvement. For example, in order to generate the signals that cause data to be read from the output buffer memory at the proper time and at the proper rate, the Parker et al. arrangement includes analog circuitry which generates ramp signals that are respectively synchronized to the horizontal and vertical timing of the television display system. These ramp signals are coupled to compartor circuits which supply an output signal that is delayed relative to the horizontal sync signal by an amount of time which will properly position the displayed image relative to the vertical boundaries of the television screen. The signals supplied by the comparator are utilized to enable a gate circuit which causes a voltage-controlled oscillator to supply the clock pulses that transfer data from the output buffer memory to the system DAC to thereby provide image data to the DAC at the proper time. In addition, to control the VCO so that the rate at which the output buffer memory is read will vary inversely relative to the y coordinate of the horizontal line associated with the image being formed, the above-mentioned ramp signal that is synchronized to the vertical sync signal is utilized as the VCO frequency control. To modulate the pulse repetition rate of the VCO in the above-mentioned manner, the frequency control signal is, in effect, multiplied by a scale factor that is stored in a read only memory and varies with the column address of the data being read from the buffer memory.
Although the above-discussed analog circuits provide satisfactory system operation, there are several advantages to be obtained with a system that employs only digital logic circuitry. In this regard, analog circuits such as those circuits utilized to generate the ramp signals in the scan converter disclosed by Parker et al. are generally more sensitive to component variations than are digital circuit arrangements and, as a result, far more extensive calibration or adjustment procedures may be necessary during the manufacturing process. Oftentimes, further component variations occur during the service life of the system and an analog arrangement may require a more extensive calibration and maintenance procedure both in the amount of testing and adjustment and in the frequency of such calibration and maintenance. Further, circuitry that is embodied in digital form is often more economical to manufacture than an analog realization of a system that performs the same function, even though the digital arrangement may include a higher parts count than an analog circuit arrangement which provides similar system performance.
Accordingly, it is an object of this invention to provide an improved digital scan converter for use with apparatus such as sector scanning surveillance systems wherein image signal information is provided that is compatible with a display device operating in a Cartesian coordinate format.
It is another object of this invention to provide a real-time digital scan converter for use with a conventional television system wherein the first portion of a signal processing method which converts signals gathered by the surveillance system in a polar coordinate format into signals compatible with the television system Cartesian format is effected by sampling the surveillance system reflection signals at a rate proportional to the cosine of the azimuthal angle of the associated scanning path and the second portion of such signal processing method is effected as stored digital signals representative of the reflection signal samples are read from a memory and converted to an analog signal.
Still further, it is an object of this invention to provide a digital scan converter of the above-mentioned type wherein the second portion of the signal processing method which converts the polar coordinate format signal information into Cartesian coordinate signal information is implemented entirely with digital circuit arrangements.
Further yet, it is an object of this invention to provide a digital scan converter, which not only meets the above-stated objects, but is of minimal structural complexity and therefore relatively economical to fabricate and maintain.